Air bubbles out the returns is a sighn of a leak on the suction side of your plumbing. Check all the fittings around the pump and filter , if everything is tight and no air leak you need to get a pressure test on the plumbing line from the skimmer to the pump.

If you are using a long plastic hose, i.e. maybe 4-6 sections for a pool sweep, check the plastic sections for holes. After a period of time, the plaster/cement in your pool will wear a hole in the plastic hose and it will suck air into the system. This has happened to me twice in the last 3 years. Good luck.